The necessary ingredients
NOVA: Why do we think Mars might have had life?
Chris McKay: Well, it's not because of anything that's there today. It's a dry, frozen desert. But there's evidence recorded on its surface that Mars at one time had water, lots of it. There were rivers, lakes, maybe even an ocean. Mars had water early in its history, possibly at the same time Earth first had life. It's that comparison—water on Earth, water on Mars, life on Earth, what happened on Mars? That's the question.
What else about Mars would support the notion that life might once have been there or might even still be there?
Mars is not that far from Earth. It's one of the terrestrial planets, along with Earth and Venus. It formed from roughly the same materials as Earth. We know that it has carbon, water, nitrogen. Right now Mars has everything needed for life except one thing—liquid water. But we see evidence that it had liquid water in the past.
In fact, there's a distinct parallel between early Mars and early Earth. Every environment that would have been on the early Earth could also have been on Mars. So wherever life made its initial evolution on Earth, that same environment should have existed on Mars as well.
Also the atmosphere?
Right. The only way to understand that Mars had liquid water in its past is to suppose that it had a much thicker atmosphere, presumably one made of carbon dioxide. Long ago Mars lost its atmosphere. Where did it go? We think that most of it is tied up now in rocks. It's been turned into carbonate. It's been mineralized.
That thicker atmosphere is needed to stabilize the water, but it would also have made Mars warmer, and it would have provided the material that life needs. Life could have taken up the carbon dioxide from that atmosphere. The atmosphere would have protected life from cosmic rays and other radiation sources, and it could have provided weather and all the sorts of things we have here on Earth.
All the conditions necessary for life.
Exactly. When we look at life on Earth, we see that life needs a series of things. It clearly needs energy, it needs carbon, it needs a few other elements. The most important requirement for life is liquid water; we think that's the defining requirement for life in our solar system. There's plenty of energy, there's plenty of carbon, there are plenty of other elements on all the planets in our solar system. What's rare and, as far as we know, only occurs now on Earth and early in Mars' history, is liquid water on the surface.
Why is carbon so important?
That's a good question, and we don't have a fundamental answer to it except that we see that all life we observe uses carbon. Carbon has some very good properties for life in terms of its ability to make molecules that link together and make polymers. Maybe other molecules could do this, too; people have speculated on silicon, for instance, which is in many ways similar to carbon. It's hard to know if carbon is really the essential ingredient for life, or just the ingredient that we happen to use here on Earth.
I'm optimistic that Mars had life.
Was there oxygen on the early Mars?
Well, when we look at the Earth, we see that through most of its history life was very small—microscopic. It's only with the rise of oxygen that we see the development of large animals and ultimately intelligence. Based on that observation, we think that early Mars probably was only microscopic as well in terms of life. But it's possible that oxygen rose more rapidly there. It's a smaller planet, and it lacks the sort of tectonic recycling that early on prevented the buildup of oxygen on Earth. Mars could have become oxygen-rich much faster than the Earth.
And that could have led to large creatures on Mars much faster than on our planet. This is speculation, of course, but it's possible that evolution on Mars went faster than evolution did on Earth. So we have to be careful when we use the Earth as the model for Mars, because the planets are different.
Chances for life
Are you saying Mars could have had complex life?
It's possible that Mars, being smaller than the Earth, evolved more rapidly than the Earth in terms of oxygen, and that if life started on Mars, it could have reached the level of complex life faster than Earth. I've done calculations that suggest it could have reached a level of complex life a thousand times faster than our planet. Instead of taking two billion years for the increase of oxygen and complexity of life, on Mars this could have happened in a few million years. So on Mars we'll look for microscopic life, but we should keep our eye open for something more interesting, more complex. It might be there. It's worth looking for.
Some scientists believe life never developed on Mars. Why do they say that?
Well, right now we have no scientific data that tells us that life did or did not develop on Mars. My intuition tells me that what life needs is water, and we see a planet that had water, so I'm optimistic that Mars had life. Other people may think that it's more difficult to start life and think that just because Mars had water, it's not necessarily probable that it had life. We really don't know. We don't know how life started on Earth. We don't know if it would have started on another planet. We don't understand the details of planetary evolution well enough. The only way to advance our knowledge is to go look on Mars.
I've heard "Yes, Mars had water, but not long enough for life to have existed."
We don't know how long Mars had water, but we also don't know how long it takes for life to evolve. Some people argue that life can start very quickly, a million years or less; some people argue that it takes billions of years. We don't know.
The one bit of evidence that we might bring to bear on this is the record of life on Earth. It appears that life started quickly here. Life seems to be present soon after the formation of the Earth billions of years ago. If that's true, then you might argue that life starts quickly. But it's hard to reach that conclusion based on only one example. If you move to California and win the lottery the first day, that doesn't mean it's easy to win the lottery.
Life may have started on Earth very quickly but purely by accident. It may be a very rare, difficult event. On the other hand, life may be easy to start under Earth-like conditions on any planet. These are questions that we'll never answer staying here on Earth. We've got to go look at another example. We've got to go see if it happened on Mars.
If we go to Mars and we find evidence for life there, a separate origin of life, I think it's clearly telling us that life starts readily on any Earth-like planet. If we go to Mars and find that it had water, it had a thicker atmosphere, it had everything needed for life, and it never developed life, then I think that would make us a little pessimistic in our predictions about life on other planets.
Why did Mars die?
Well, suppose you were on Mars three to four billion years ago, and you were walking around on this very nice world with a thick atmosphere and water, and everything was just fine. Well, gradually things would start getting worse and worse and worse. What's happening, you'll notice, is that the atmosphere is getting thinner.
Meanwhile, your friends on Earth would be finding that their planet was just fine, that there was recycling due to plate tectonics, that the Earth was maintaining its atmosphere. So the two planets start off the same, one goes down, and the other maintains itself. That's the fundamental difference between the history of Mars and of Earth.
It's possible that there are still places on Mars today where life is a going thing.
What happens when you lose your atmosphere?
Well, the main problem of living without an atmosphere is that there's no greenhouse effect. It's very cold. Everything freezes. And the pressure is so low that water goes directly from solid to vapor without forming a liquid. So this is a double whammy from the point of view of life—temperature and pressure too low for liquid water.
Plus lots of nasty ultraviolet radiation.
Yes. There is both UV and cosmic radiation coming through the thin atmosphere and hitting the surface. But those are not really powerful detriments to life. Ultraviolet light and cosmic radiation are bad for humans, of course; we would have higher incidents of cancer and so on. But many organisms on Earth have learned to cope with UV radiation—microorganisms in particular. And organisms that live in the subsurface have no worries about that sort of radiation.
I don't think that the radiation itself would prevent life on Mars if the atmosphere was thicker and if liquid water could be present. Life can figure out a solution for everything else, but liquid water seems to be the one thing that life can't work around.
Weighing the evidence
Are there any recent findings to support the possibility of past life?
The most interesting recent results from Mars all focus on water. From the Odyssey spacecraft we now have direct evidence that Mars has massive ice in the polar regions, in the permafrost there. Also, there is clear evidence of ancient rivers and channels being carved by water. The more we learn about Mars, the more we're convinced that it was a water planet.
Is there evidence that liquid water might still exist there?
There is some evidence that suggests there is still some activity that could be related to the presence of liquid water, or the melting of snow, or the melting of ice in recent times. That evidence is very interesting, but it's still controversial.
Might the Mars Exploration Rover scientists find signs of ancient life, say, in Gusev Crater?
We're pretty sure Gusev Crater was full of water. It was really a crater lake. The idea is that maybe if there was life in that lake and it died and settled to the bottom, it's preserved in the sediments as fossils. We might find fossils right there on Mars, and that would be interesting.
What I'd like to do next is then go down into the ancient terrain near the south polar region in the permafrost there and drill and try to find not a fossil but an actual dead martian organism frozen in the ground, a corpse, something we could do an autopsy on.
Any possibility there could be life still extant?
My guess would be that if Mars had life in its early history that it has all died out, but we're not sure. It's possible that there are still places on Mars today where life is a going thing, say, near the polar regions, where there are possibilities for water from the melting of ice or more likely deep underground, where geothermal heat from the interior of the planet may be enough to keep the water liquid. Those are the possibilities for life today. I'm not optimistic.
I think sending humans to Mars is a possible task.
You mentioned geothermal heat, but earlier you said Mars has lost its heat.
The geothermal heat on Mars is much lower than that on the Earth, but it's still there. If you were to drill down a kilometer or two below the surface, it would become warm enough that the ice would melt.
Could microbes from Mars' early history that are frozen into the subsurface potentially still be viable?
Well, imagine in the permafrosts on Mars a bug frozen into the ground, waiting for things to warm up. How long might it survive? We think the answer might be hundreds of millions of years. Unfortunately, on Mars they may have been waiting for several billion. So even for these guys it may have been too long a wait. But we're not sure of that. We should be prepared for the possibility that we'll go to Mars, we'll dig up bugs, and they'll still be viable.
Costs of exploration
Given your hopes for the polar regions, it must have been extremely disappointing when the Mars Polar Lander vanished in 1999.
Indeed. It was going to land down near the south polar cap, down in that ice-rich material that may hold the organic or even biological record of life on Mars. So needless to say, we were very disappointed when it crashed, not just for the loss of the mission and the loss of the time and effort that went into it, but for the loss of the opportunity to advance our scientific understanding of Mars in that way.
But that's just the way it is when you explore planets. They're far away, and it's hard to make sure things are working without someone there to fix them. On average, only one out of three of the missions that we Earthlings have sent to Mars have succeeded. The odds are not good, but that's just the cost of doing this kind of exploration. It's like the major leagues. If you're batting 300, that's pretty good. We're batting 300.
Why is it so difficult to get a mission, go to the planet, and dig up this stuff?
It's difficult to go to Mars. It's a long way away. If you send a robot and something goes wrong, there's no one there to fix it. If you send humans, you've got to make sure they have enough food and water and air to make it there and back. It's a challenging prospect.
We've sent people to the moon. What's so hard about sending them to Mars?
Well, imagine you were going to send a well-trained scientific team to search for life on Mars. It would take them at least six months to get there, and on the way you'd have to make sure that their bones and muscles didn't get weaker in the microgravity of space. Once they got there, of course, anytime they went outside they'd have to wear a spacesuit. Pressure, oxygen, food, water—everything would have to be provided for their entire trip.
Nonetheless, I think sending humans to Mars is a possible task. We know how to do it. We have the technology. If we wanted, we could set up a research station on Mars and do the scientific exploration that would answer these questions. But I think it's still some time in the future before we do that.
If there was a mission tomorrow, would you want to be on it?
If there was a mission going tomorrow to Mars and they were looking for somebody to go out in the field and dig for fossils, I would volunteer. Why not? As long as they promised to bring me back after a few years!